A variety of electronic devices such as laptop computers, personal digital assistants, cell phones, etc., may be powered by a battery pack. The battery pack may include multiple rechargeable battery cells connected in series. The battery pack may be recharged when a suitable DC power source, e.g., an AC/DC adapter, is coupled to the electronic device. The electronic device may include a battery gas gauge to calculate a remaining capacity in the battery pack such that a user can decide when to recharge the battery pack.
FIG. 1 shows a block diagram of a conventional battery gas gauge 100. In the example of FIG. 1, the battery pack includes two series-connected cells, the cell 102-1 and the cell 102-2. The battery gas gauge 100 includes an analog-to-digital converter (ADC) 108 coupled to the cell 102-1 and the cell 102-2 through a voltage divider 106 for measuring an open circuit voltage (OCV) of the battery pack. An open circuit voltage of a battery pack refers to a voltage across the battery pack when the battery pack is disconnected (neither charging nor discharging). The open circuit voltage of the battery pack can be measured after the battery pack is disconnected for a time period long enough such that the voltage across the battery pack becomes relatively stable.
The measured open circuit voltage Vbat of the battery pack is stored in a register 110. A processor 112 reads Vbat from the register 110 and determines a relative state of charge (RSOC) based on a predetermined OCV table 114. More specifically, the processor 112 divides Vbat by the total number of cells in the battery pack to obtain an average cell voltage, and looks up the OCV table 114 to determine a relative state of charge of the battery pack.
The OCV table 114 is a lookup table that can indicate a relationship between the average cell voltage and corresponding ROSCs. Table 1 shows an example of an OCV table. A set of relative state of charges corresponding to a set of average cell voltages are stored in the OCV table in advance.
TABLE 1RSOC(%)100959085807570656055OCV(mV)4160410040624036400539653922388338483814RSOC(%)5045403530252015105OCV(mV)3773373436953660362335883557352034853374
For example, if the open circuit voltage Vbat of the battery pack is 7.4V, the processor 112 can calculate an average cell voltage equal to 3.7V. In the OCV table, an open circuit voltage of 3695 mv corresponds to a relative state of charge of 40%. Therefore, the processor 112 determines that the battery pack has a relative state of charge around 40%. If the battery pack has a rated full capacity level of 1000 mAh, the remaining capacity of the battery pack is determined to be around 400 mAh.
Therefore, in the conventional battery gas gauge 100, an average cell voltage is used to determine a relative state of charge of the battery pack. However, charging and discharging of the battery pack through normal operation over time may result in cell-to-cell variations in cell voltages. When one or more cells in a series string charge faster or slower than the others, an unbalanced condition may occur. Thus, cell voltages of the cells in the battery pack may be different. For example, when Vbat is 7.4V, the voltage of the cell 102-1 can be 3.623V, and the voltage of the cell 102-2 can be 3.777V. When the battery pack discharges, cell voltages of the cell 102-1 and the cell 102-2 both decrease. The battery pack may be prevented from discharging if any cell voltage decreases to a protection threshold, e.g., 3V. Since the cell 102-1 has a cell voltage lower than the cell 102-2, the battery pack will be prevented from discharging if voltage of the cell 102-1 decreases to 3V. However, at this time, the cell 102-2 may have a voltage higher than 3V. In other words, there is some capacity of the cell 102-2 that is not utilized, which can result in a difference between an actual full capacity level of the battery pack and the rated full capacity level of the battery pack. Thus, the accuracy of the remaining capacity of the battery pack measured by the conventional battery gas gauge 100 may be reduced.